This invention relates generally to collecting and distributing apparatus or underdrains which are part of liquid purification systems.
One method of purifying liquid uses filtration systems with filter beds having one or more layers of material. The top layer consists of a granular media which is made up of fine particulate matter such as anthracite, sand, carbon, or garnet. The next level below the granular filtration media comprises support or packing gravel. Underdrain laterals are placed below the layer of gravel. These are long narrow channels which are laid laterally across the width or length of the filter bed. A plurality of such underdrain laterals are placed side by side, so that the entire lower portion of the filter bed beneath the gravel layer is composed of the laterals.
The liquid to be filtered is applied across the top of the granular layer. As it seeps through the granular layer, waste material removed from the liquid accumulates and adheres to the particles of the granular layer. The liquid then flows through the granular layer through openings in the top of the underdrain laterals and then through a flume beneath the underdrain, through which the filtered liquid is discharged.
To maintain the efficiency of the filtering system, it is necessary to periodically clean the waste material from the granular and gravel layers. This is accomplished by the use of backwash water, which flows in the reverse direction through the filtration system. The backwash water is introduced at the flume beneath the underdrain. It flows upward through the underdrain into and through the gravel layer and the granular layer, from whence it is discharged.
In order to make the backwash process more efficient, a gas such as air is often used. The purpose of the gas is to sufficiently agitate the gravel and granular material to loosen and free waste material which has adhered during the filtering process. During the gas cycle, a water level slightly above the top of the granular level is maintained. After and or during the gas cleansing cycle, the backwash water is introduced to remove the waste material which has been loosened and freed by the gas.
Two approaches are used with regard to the cleansing of the filter media. In one approach, the gas cycle is used first and immediately followed by a cycle during which backwash water alone is used. Another approach is to use the gas cycle first and immediately follow with a cycle during which both gas and backwash water are introduced and flow through the filtration system simultaneously.
The use of the combined cycle of gas and backwash water is more efficient since the amount of water required is drastically reduced as compared to the backwash water required with a second cycle of backwash water only. Furthermore, the combination of gas and water for the second cycle provides a more efficient cleaning operation during the use of water alone. However, when the granular material is very fine, the combined cycle cannot be used because some granular material is carried away by the gas bubbles in the backwash water and lost.
A typical operation might be the use of gas only in the order of 3-5 standard cubic feet per minute per square foot of the filter bed 2-5 minutes. Then the gas at 2-5 standard cubic feet per square foot per minute is mixed with water at 5-71/2 gallons per minute per square foot for 2-5 minutes. If the second cycle is backwash water only, up to 30 gallons per minute per square foot 3-5 minutes might be required depending on the filtration media.
The filter bottom of M. L. Stuppy, U.S. Pat. No. 3,110,667 specifies a block with two lower chambers alongside each other and two upper chambers, each one above a lower chamber. Ports between the lower chamber and the upper chamber provide compensation which assist in evening out the pressure distribution of the backwash water and lowering the amount of head pressure required. Stuppy however, does not provide for insertion of gas such as air to assist in the backwash process. The only way that gas can be applied in the backwash process in Stuppy is to add a network of pipes to supply gas above the granular material level, which would be prohibitively expensive, or to add a network of pipes above the underdrain which would disturb the granular material and cause it to mix with the gravel. The finer granules could then clog the ports at the underdrain and seep into the underdrain. Maldistribution of granular material across the filter bed could also result.
Farrabough, U.S. Pat. No. 4,065,391, discloses an underdrain which provides for the use of the gas for cleansing. Chambers are formed by diagonal walls. This results in alternate chambers that mix gas and water and chambers that carry water only, with compensating orifices in the diagonal walls.
Sassano et al., U.S. Pat. No. 4,214,992 specifies a block with an air and water chamber in the center and water dispensing chambers on both sides. The chambers are formed with diagonal and straight walls and are rhombic in shape.
A major problem with the underdrains of Farrabough and Sassano is that when the gas and water mix in the chamber during the gas cycle, turbulence is often set up which creates standing waves which can become destructive. The turbulence often result in the mixing of the granular particles with the gravel, (six layers of gravel are required with Farrabaugh and Sassano underdrains), and the seepage of granular particles into the chambers which clog the orifices on the top of the underdrain and in the diagonal walls of the underdrain.
If the turbulence becomes severe enough, catastrophic damage can occur and has occurred, causing poor distribution of water and gas, damage to and breaking up of the underdrain, and lifting of the underdrain blocks or laterals off the filter bed floor.
Furthermore, it is not possible to connect the gas chambers from one underdrain lateral to another with cutouts to equalize the gas distribution. It is also not possible with the underdrains of Farrabough and Sassano to interconnect with cutouts the water chambers from one underdrain lateral to another to equalize water distribution.